HVAC523

Heat Sources

Classification of Hydronic heat sources

Conventional gas and oil fired boilers Condensing gas fired boilers Electric resistance boilers Electric thermal storage equipment Hydronic heat pumps Renewable energy heat sources

Heat sources

Of these, conventional gas and oil boilers are currently being used in the vast majority of residential and light commercial hydronic systems in the USA.

Gas and Oil fired designs

Most boilers that operate on natural gas or fuel oil can be classified according to their physical construction and heat exchanger material.

Cast Iron Sectional Boilers

The cast-iron sectional boiler is currently the most common type of boiler used in residential and light commercial buildings in North America.

Four section boiler

Cast Iron Sectional Boilers

Since the water flows completely around the heat source, these types of boilers are called Wet Base boilers.

The wet base boiler is the common configuration for oil fired boilers.

Cast Iron Sectional Boilers

In this type of boiler, water is heated in cast iron chambers called sections.

The sections are bolted together to form a boiler block.

The hot gases generated in the combustion chamber near the bottom of the boiler rise up through the cavities between the sections and transfers the heat to the water within.

Cast Iron Sectional Boilers

The greater number of sections, the greater the heat output of the boiler.

Most cast iron sectional boilers for residential and light commercial applications are sold as packaged boilers.

Cast Iron Sectional Boilers

This implies that components such as the circulator, burner and controls are selected and mounted on the boiler by the manufacturer.

The components supplied by the manufacturer assume the boiler will be used in a typical residential system.

In some cases one or more of these components may not be best for a specific application.

Packaged boiler

Cast Iron Sectional Boilers

The amount of metal in cast iron section boilers makes them relatively heavy.

Even small cast iron boilers can weigh as much as 300 to 400 pounds.

Such boilers typically contain 10 – 15 gallons of water.

Cast Iron Sectional Boilers

The combination of metal and water weight gives cast iron boilers the ability to absorb a significant amount of thermal energy.

Such boilers are said to have a high thermal mass.

These boilers are known as high mass boilers

Cast Iron Sectional Boilers

A well insulated boiler housing, or jacket as it is commonly called, reduces standby heat loss from the boiler.

Cleaning Cast Iron Sectional Boilers

Some manufactures design the cast iron boilers for vertical passage of the flue gasses.

To clean these types of boilers, the sheet metal top panel, flue connection and flue gas collector assembly must be removed before a cleaning brush can be maneuvered between the sections.

Cast Iron Sectional Boilers

Another type of cast iron boiler design uses horizontal passages between the sections with a hinged door to access both the combustion chamber and the flue gas passages.

Steel Fire Tube Boilers

Steel has been used in the construction of boilers for decades.

In a steel Fire Tube boiler, water surrounds a group of steel tubes through which the hot combustion gasses pass.

Steel Fire Tube Boilers

Spiral shaped baffles known as turbulators are inserted into the fire tubes to increase heat transfer by inducing turbulence and slowing the passage of the exhaust.

The fire tubes are welded to steel bulkheads at each end to form the overall heat exchanger assembly.

Steel Fire Tube Boilers

Although steel fire tube boilers generally have less mass (metal weight) than cast iron boilers of similar capacity, they often hold more water.

A typical steel fire tube boiler contains between 15-30 gallons of water.

Steel Fire Tube Boiler

Copper Water Tube Boilers

Some boilers use water filled copper tubes as their heat exchanger.

These also come in vertical and horizontal tube arrangements.

Copper tubes are usually manufactured with fins that greatly increase the are exposed to the flue gasses for improved heat transfer.

Copper Water Tube Boilers

Combined boiler /domestic hot water tank assemblies

A large percentage of residential hydronic systems supply both space heating and domestic hot water.

Although several methods have been used to provide domestic hot water from a space heating boiler, it is generally accepted that the most efficient approach uses a storage tank that is indirectly heated by the boiler.

Space heating boiler with an indirect fired water heater

Combined boiler /domestic hot water tank assemblies

Several manufactures now offer products that combine a boiler and hot water storage into a single unit.

Combination unit

Dry Base Boilers

Another common configuration is called the Dry Base Boiler.

These boilers suspend the water filled heat exchanger above the combustion chamber

Dry Base Boilers

Gas fired cast iron boilers and oil fired vertical tube boilers are typically designed as dry base boilers.

Cast Iron Boilers

All the boilers listed above (except the combo unit) are classified as conventional boilers.

Conventional boilers usually operate with a maximum 85 percent efficiency.

Conventional boilers are designed to operate with a hot enough exhaust temperature so that the byproduct of combustion, water vapor, will not condense back into a liquid.

Cast Iron Boilers

If the exhaust temperature were to be lowered and the water vapor were to reach its dew point, other byproducts of combustion would be entrained in this fluid and would create a very caustic environment for the cast iron or steel the unit is made out of.

Condensate

A good analogy to flue gas condensation in a boiler is seen in the exhaust from a car on a cool day.

When the car is first started, the temperature of the exhaust pipe and muffler is low enough to cause some of the water vapor produced in the engine to condense by the time it reaches the tailpipe.

Condensate

Water can often be seen dripping from the tailpipe when the car is first started.

After the tail pipe warms above the dew point temperature, the water vapor no longer condenses, and the dripping stops.

Condensing Boilers

After the energy crisis of the 1970s, higher fuel costs motivated boiler manufacturers to research methods for improving boiler efficiency.

Analysis by engineers and scientist found that significant amount of heat can be recaptured from the exhaust stream if the water vapor it contains can be condensed within the boiler.

Condensing Boilers

Condensing boilers are designed with large heat exchanger surfaces to extract more heat from the exhaust gasses compared to conventional boilers.

The heat exchangers are made out of stainless steel or some kind of protected metal.

Condensing boiler

Condensing boiler

Condensing boiler

Currently available boilers use high grade stainless steel or proprietary electroplated coatings to effectively eliminate corrosion.

Condensing boilers are more expensive than conventional boilers of equal capacity.

Condensing boiler

Condensing boilers that are designed and installed properly, will achieve much more efficiency than conventional boilers.

When designed and installed properly these types of boilers can attain 95+ percent efficiency.